1. Field of the Invention
The present invention relates to systems and methods for profiling surfaces of an object such as found in a cylindrical container and, in particular, the present invention relates to a profiling measurement system for beverage cans that utilizes both non-contact optical profiling and contact mechanical gauging to provide overall sidewall surface and thickness information for the beverage can as well as diameter, tilt, ovality and squarity.
2. Statement of the Problem
There are needs in industry to provide a three dimensional profile of the surfaces of cylindrical containers to ascertain the conformance of cans to design specifications, to aid in the understanding of the various manufacturing or shipping processes on the container, to analyze the material properties of the container, and/or to track the variations in can configuration caused by the manufacturing processes involved.
For example, aluminum beverage cans are containers having a convex surface around their entire outer cylindrical sidewall which may be as thin as 0.003 inches. Conventional surface evaluation techniques can provide precise measurements of unfilled beverage cans utilizing contact techniques, but such techniques are principally limited to discrete measurements. Furthermore, it is difficult to repeat measurements. Hence, a need also exists to evaluate the sidewalls of aluminum beverage cans and to provide repeatable metrology of the entire can sidewalls so that information concerning the process can be obtained. Such information provides valuable feedback in understanding, evaluating, and adjusting the manufacturing processes to form the can.
While the present invention is generally directed towards the surface profiling and thickness measuring of aluminum beverage cans, it is to be expressly understood that any convex surface or portion thereof of a container, work piece, or object could be likewise evaluated under the teachings of the present invention. In U.S. Pat. Nos. 4,872,757 and 4,863,275 commonly assigned to the assignee of the present invention, an optical convex surface profiling engaging apparatus and method is set forth. The present invention utilizes and improves upon the teachings of the optical profiling instrumentation of U.S. Pat. No. 4,863,275 to provide optically generated data of the present invention.
The teachings of the '275 and '757 patents are improved upon by reducing noise and providing higher resolution in the optical path and by adding a separate mechanical contact measurement system which contacts the inside and outside surfaces of the container to provide a thickness profile circumferentially around the container. The contact gauging approach is designed so as to perform its gauging without deflection or deformation of the container sidewalls. A need exists, therefore, to improve upon the resolution and to lower the noise of the '275 and '757 approaches. A need further exists to provide a separate sidewall thickness data input from a mechanical gauging system so as to generate a combined thickness dimension to the overall surface profiling of the container so that an overall topological profile of the container can be generated.
A need also exists to automatically obtain a plurality of precision measurements of circumferential sidewall thicknesses at a number of elevations, the dome height, overall height, tilt, ovality, squarity, and the surface profile of a container. In addition, a need further exists to make the combined optical and contact profiling approaches easy to calibrate and to autozero the system for each new container being profiled.
A further need exists to make the overall combined profiling system based upon the use of a simple computer such as a personal computer which can correlate the two independent sources of contact and non-contact data together so as to provide a correlated output containing the measured physical characteristics of the profiled container.
A need exists to provide the mechanical gauging of the thickness with a transducer that performs the actual thickness measurements without deforming the surface of the sidewalls of the container.
Finally, a need exists to provide a system rugged and reliable enough to provide precision container measurements on the factory floor as well as one precise enough for critical measurements in the laboratory.
3. Prior Art Patented Approaches
A number of needs therefore exist for a profiling measurement system that can be used in the manufacturing environment for surface and thickness profiling of containers such as aluminum beverage cans and for the detection of defects therein.
U.S. Pat. No. 4,629,319 by Clarke et al. sets forth an invention for the electro-optical sensing of defects such as dents, creases, low spots and flat spots on the surface of sheet metal or plastic panels such as those used on the hoods and fenders of cars, refrigerators and furniture. Clarke directs light onto the surface. The light reflected from the surface impinges upon a retro-reflective member to return the reflected light to the surface area to be re-reflected. The re-reflected light is then imaged and carries information as to the nature of the defect.
U.S. Pat. No. 4,326,808 issued to Pryor et al. sets forth an apparatus for determining defects in the outer surface of an elongated object wherein the object to be inspected passes through an aperture of a conical mirror surface. The light is directed onto the mirror surface, reflected by the object, imaged and then analyzed to determine the nature of the defect.
U.S. Pat. No. 4,675,730 issued to Adomaitis sets forth an apparatus for continuously inspecting the surface of a moving object for defects. The surface of the object is illuminated with both specular and/or diffused light of selected wavelengths. The moving surface is rendered momentarily motionless and a plurality of sensors located to view the width of the object detects the presence of the defects. An electronic image of the defect contains gray scale levels that represent varying intensities of the light reflected by the defect. A comparison is then made with a defect free image and, if different, then a freeze frame analysis of the object is made.
U.S. Pat. No. 4,410,278 issued to Makihira et al. sets forth an apparatus for inspecting the outer peripheral surface of a cylindrical object. The light, in slit form, is projected on the surface of the cylindrical object. The reflected light is detected by a photo detector and is quantitized at threshold values higher or lower than an average level. The three types of surface defects of a chip, a crack, and a pit are separately detected and identified.
U.S. Pat. No. 4,226,539 issued to Nakagawa et al. also sets forth a system for cylindrical body surface inspection. The cylindrical body is rotated around its axis at a constant speed. Light is directed onto the surface of the body and an optical detector detects the reflected light indicative of a surface condition of a small width baseline which is parallel to the axis of the cylindrical body. A sampling detection repeats as the body rotates to scan the entire surface of the cylinder.
U.S. Pat. No. 4,162,126 issued to Nakagawa et al. sets forth a camera system which senses diffused reflected light from the surface of an object in order to analyze the reflected light wherein a threshold level is used so that surface defect patterns such as a broken cavity, a pit, or a crack pattern can be selectively discriminated.
All of the above represent prior approaches for surface profiling and gauging involve a non-contacting system like the present invention. However, each of these approaches analyzes light which is reflected from the surface and they are suitable for evaluating convex surfaces. The present invention does not analyze reflected light; rather, it evaluates a shadow edge from the container as it is being rotated.
The following prior art approaches set forth inspection techniques which analyze the shadow of the object being inspected.
U.S. Pat. No. 4,576,482 issued to Pryor sets forth an apparatus for determining accurate dimensions of individual work pieces. The apparatus is a non-contacting system wherein a collimated or semi-collimated light source illuminates at least one edge of the work piece with parallel light rays so that a lens can form an image of the illuminated edge. This image provides an average shadow over an area of the edge. An array of photosensitive elements such as photodiodes produce an electrical signal in response to the light impacting thereon. The edge image as sensed by the photodiode array can then be analyzed to provide a determination of a dimension such as length, squareness, curvature and the like.
The 1972 patent to Hemsley (U.S. Pat. No. 3,666,885) utilizes a strobe for directing a short duration light pulse onto the object to form a shadow image of the object in a camera. The camera is modified for single line scanning and produces a line waveform of the shadow image. The Hemsley approach is adaptable for either hot or cold objects being scanned. Hemsley utilizes a referenced object to determine the initial measurement and then compares the object being inspected to the measured value and records any differences in the line waveforms.
The 1984 patent to Forbes (U.S. Pat. No. 4,465,937) utilizes a light source mounted in a scanning head that is rotated about the object and which can be advanced along the length of the object to provide data pertaining to the entire peripheral surface of the object. Forbes utilizes a light source that provides a beam of light having a width greater than the width of the object being scanned so that as the light source is rotated around the object, deviations in the shadow's edge can be sensed by photo sensors and determined.
The 1984 patent to Daudt (U.S. Pat. No. 4,476,533) pertains to a non-contact optical gauger for measuring hot glassware articles while they are being manufactured. The system makes specific measurements of height, perpendicular, neck diameter, and height variation of the glassware article. Articles falling outside predetermined measurements are rejected.
The METOP.RTM. computerized front end and back end stations are available for measuring sidewall thicknesses of two piece cans. The METOP.RTM. system is manufactured by Metop, Box 836, S-20180 Malmo Sweden. The METOP.RTM. system is not suitable for automatic measurement since it requires hand loading of the cans. Because of the hand loading, the operator in handling the can may deform the can or portions thereof. In addition, once a measurement is made and the can removed, a repeat measurement at the same location is extremely difficult to obtain.
4. Solution to the Problem
The above prior art approaches do not set forth a combined profiling system designed for factory operation which utilizes data generated from a non-contact system and data from a contact system for providing overall sidewall information correlating the two independently generated sets of data together. The outputted sidewall information of the present invention includes a solid model of the container sidewall and container diameter, tilt, ovality, squarity and/or height. In the case of a beverage can, the dome height is also provided. The present invention, therefore, substantially improves upon the above prior art approaches.
The present invention performs both contact and non-contact measurements on containers that are placed on a rotating mandrel that can be selectively raised and lowered. In the lower position, the container is positioned within the optical train of the non-contact measurement apparatus wherein a plurality of measurements of the container's edge are made during a first time interval so as to construct a surface profile of the sidewall of the container. The present invention improves upon the surface profile information by using a digital camera that digitizes directly to the signals from each pixel based upon a shadow edge. The present invention significantly improves upon the precision of the sub-pixel interpolation algorithm used to find the shadow edge of a container image.
The mandrel is then raised to the second contact measurement position during a second time interval. A plurality of sensor mechanisms selectively engage the sidewalls of the container through pneumatic actuation. The container is then rotated and each sensor is sampled a predetermined number of times to obtain a plurality of sidewall thicknesses at different elevations. A unique balanced double-pivot scissors arrangement is used in each sensor mechanism to make the sidewall thickness measurement. The balanced double-pivot scissors mechanism is pivotally mounted to permit free rotation so as to accommodate varying radial diameters of the container. A second pivot concentric to the first pivot permits the arms of the scissors to open and close about the sidewall of the container. The balanced double-pivot scissors mechanism is designed to minimize errors caused by environmental temperature changes.
The present invention utilizes a unique autocalibration technique before the sidewall measurements of each container are made.